Fiber-based optical communication systems have rapidly increased in importance in recent years. The ability to transmit large volumes of information over optical fibers has resulted in increasing demands for systems which can take advantage of the bandwidth available in optical communications systems. One problem in the art which has been recognized and partially addressed in the past is the need to efficiently shift the wavelength of optical signals used to carry information. Wavelength shifters are often required when a signal is to be routed to a subnetwork existing within a larger network, and for various types of optical processing.
A variety of optical wavelength conversion techniques have been proposed, but each presents one or more problems. A review of the known techniques is given in a paper by S. J. B. Yoo, Journal of Lightwave Technology, vol. 14, n. 6, June 1996, pag. 955-966.
Some systems use optical/electrical solutions, such as electrically detecting the information carried by a first optical signal and then modulating a second laser signal using the information contained in the first signal. This is disclosed, for example, in U.S. Pat. No. 5,267,073 (Grasso et al.). This technique is inherently limited by the speed of the associated electronics, and may be not transparent to bit rate.
Other systems use cross-gain modulation (XGM) or cross-phase modulation (XPM) in semiconductor optical amplifiers (SOA). These techniques are described, e.g., in a paper by T. Durhuus et al., Journal of Lightwave Technology, vol. 14, n. 6, June 1996 pag. 942-954 or, e.g., in the paper "An All-Optical Wavelength-Converter with Semiconductor Optical Amplifiers Monolithically Integrated in an Asymmetric Passive Mach-Zehnder Interferometer", IEEE Photonics Technology Letters, Vol. 7, No. 10, October 1995. A semiconductor interferometric optical wavelength conversion technique is described in EP 717,482 (AT&T). Wavelength converters employing SOAs are limited, among others, in the operational bit rates and in their noise performances.
Another known technique for wavelength conversion exploits four wave mixing of an input signal and a pump signal in a nonlinear medium to produce a conjugate signal to the input signal, wherein the conjugate signal is at a frequency shifted from that of the input signal. This technique is illustrated and discussed, for example, in U.S. Pat. No. 5,619,368. In an embodiment, a pump signal and an input signal co-propagate in both a clockwise and a counterclockwise direction in a nonlinear optical medium, such as an optical fiber loop mirror, in order to generate the output signal of interest. The clockwise and counter-clockwise components of both the injected input signal and pump signal are phase matched in the mirror loop by appropriate choice of fiber length, fiber dispersion zero and frequency separation.
EP 697 939 discloses a wavelength converter comprising a nonlinear optical device having a first input for a constant light of a first wavelength .lambda..sub.s, a second input for a second signal of a second wavelength .lambda..sub.p, modulated with an information, and a first output for a signal of the first wavelength .lambda..sub.s which is modulated by said information. The nonlinear optical device can comprise one of the following: a nonlinear fiber optic Mach-Zehnder interferometer, a nonlinear optical loop mirror, or a nonlinear fiber optic directional coupler.
Wavelength converters employing a nonlinear optical loop mirror are discussed in the above cited paper by S. J. B. Yoo. In FIG. 6(b) a nonlinear optical loop mirror using an optical fiber as a nonlinear medium is shown. A probe beam is split in two by a 50:50 fiber coupler and propagates in both directions. In the absence of nonlinear interaction, the output port sees no probe beam. An input signal is coupled into the loop via a fiber coupler and propagates in a counter-clockwise direction. This signal modulates the optical index of the nonlinear optical fiber owing to an optical Kerr effect, and causes the phase of the probe beam propagating counter-clockwise to increase relative to that of the clockwise beam. Due to this asymmetry, the output port sees the probe beam. Due to a finite propagation time through the nonlinear element, the probe signal is pulsed (clock) and needs synchronization with the input signal. All-fiber systems require more than 2 km of optical fibers, and unstable output can be caused due to local index variations in the fiber.
U.S. Pat. No. 5,111,326 discloses an integrated Kerr shutter and a high speed modulated optical source. A pulsed pump signal changes the polarization of a CW probe signal in a polarization maintaining fiber by the optical Kerr effect. The probe signal is decoupled at the output end of the polarization maintaining fiber and passed through an analyzer yielding an output probe signal having a wavelength of the probe signal and a pulse rate of the pump signal.
A paper by H. K. Lee et al., IEEE Photonics Technology Letters, Vol. 7, No. 12, December 1995, pp. 1441-1443, discloses a walk-off balanced nonlinear fiber loop mirror-type all-optical switch for 10 Gb/s signals of 1.3 .mu.m wavelength with 1.5 .mu.m control pulses. The disclosed nonlinear fiber loop mirror is composed of three fiber couplers and a 500-m-long polarization maintaining fiber. The signal source is a gain-switched laser diode emitting at 1.313 .mu.m. The source pulses are interleaved to produce a 10 GHz pulse train, split by a fiber coupler and then introduced in opposite directions into the nonlinear fiber loop mirror. The control beam is generated from a pulsed 1.535 .mu.m DFB laser, whose pulses are amplified and compressed. The control beam is then split into two by a 3-dB coupler and coupled into the nonlinear fiber loop mirror in directions counterpropagating to each other. One of the control beams is delayed by a variable fiber delay line with respect to the other control beam. The switching window size is controllable by controlling the relative timing delay between the two counterpropagating control beams.
A paper by G. R. Williams et al, Optics Letters, Vol. 10, No. 16, August 1995, pp. 1671-1673, discloses a soliton logic gate using a low birefringence fiber in a nonlinear loop mirror.